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Out of the box, webpack only understands JavaScript and JSON. Loaders let it handle other file types by transforming their contents into modules that webpack can add to the dependency graph. See Concepts: Loaders for how they fit into a build.

The loaders below are maintained by the webpack organization.

LoaderDescription
sass-loaderCompile Sass and SCSS to CSS
less-loaderCompile Less to CSS
stylus-loaderCompile Stylus to CSS
postcss-loaderProcess CSS with PostCSS
coffee-loaderCompile CoffeeScript to JavaScript
thread-loaderRun the loaders that follow it in a worker pool

webpack now covers what these loaders did, so their repositories are archived and they are no longer documented here.